It never ceases to amaze me how old and varied ancient trade routes were. Could someone give me an overview of what was happening between the indigenous tribes of Britain, amongst themselves and with the continental mainland before and around the time of the Roman invasion? Diplomacy, treaties, trade? I'm particularly interested in the Brigantes if that helps narrow things down.
There is actually quite a good book about this. I decided to hold off in case an expert on the subject wanted to weigh in. It's called Coins and Power in Late Iron Age Britain. It talks about how coins represent relationships of power and that Rome was already developing relationships with local rulers as they did with other regions just outside their direct control. Sorry I can't be of more help, hopefully your library can get you this book.
http://www.worldcat.org/title/coins-and-power-in-late-iron-age-britain/oclc/56340368